Conductive paste composition and semiconductor devices made therewith

ABSTRACT

A conductive paste composition contains a source of an electrically conductive metal, an alkaline-earth-metal boron bismuth oxide, and an organic vehicle. An article such as a high-efficiency photovoltaic cell is formed by a process of deposition of the paste composition on a semiconductor substrate (e.g., by screen printing) and firing the paste to remove the organic vehicle and sinter the metal and alkaline-earth-metal boron bismuth oxide.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/911,026, filed Dec. 3, 2013 and entitled “Conductive PasteComposition and Semiconductor Devices Made Therewith,” which isincorporated herein in its entirety by reference thereto.

FIELD OF THE INVENTION

The present invention relates to a conductive paste composition that isuseful in the construction of a variety of electrical and electronicdevices, and more particularly to a paste composition useful in creatingconductive structures, including front-side electrodes for photovoltaicdevices.

TECHNICAL BACKGROUND OF THE INVENTION

A conventional photovoltaic cell incorporates a semiconductor structurewith a junction, such as a p-n junction formed with an n-typesemiconductor and a p-type semiconductor. For the typical p-baseconfiguration, a negative electrode is located on the side of the cellthat is to be exposed to a light source (the “front” side, which in thecase of a solar cell is the side exposed to sunlight), and a positiveelectrode is located on the other side of the cell (the “back” side).Radiation of an appropriate wavelength, such as sunlight, falling on thep-n junction serves as a source of external energy that generateselectron-hole pair charge carriers. These electron-hole pair chargecarriers migrate in the electric field generated by the p-n junction andare collected by electrodes on respective surfaces of the semiconductor.The cell is thus adapted to supply electric current to an electricalload connected to the electrodes, thereby providing electrical energyconverted from the incoming solar energy that can do useful work.Solar-powered photovoltaic systems are considered to be environmentallybeneficial in that they reduce the need for fossil fuels used inconventional electric power plants.

Industrial photovoltaic cells are commonly provided in the form of astructure, such as one based on a doped crystalline silicon wafer, thathas been metalized, i.e., provided with electrodes in the form ofelectrically conductive metal contacts through which the generatedcurrent can flow to an external electrical circuit load. Most commonly,these electrodes are provided on opposite sides of a generally planarcell structure. Conventionally, they are produced by applying suitableconductive metal pastes to the respective surfaces of the semiconductorbody and thereafter firing the pastes.

Photovoltaic cells are commonly fabricated with an insulating layer ontheir front side to afford an anti-reflective property that maximizesthe utilization of incident light. However, in this configuration, theinsulating layer normally must be removed to allow an overlaidfront-side electrode to make contact with the underlying semiconductorsurface. The front-side conductive metal paste typically includes aglass frit and a conductive species (e.g., silver particles) carried inan organic medium that functions as a vehicle for printing. Theelectrode may be formed by depositing the paste composition in asuitable pattern (for instance, by screen printing) and thereafterfiring the paste composition and substrate to dissolve or otherwisepenetrate the insulating anti-reflective layer and sinter the metalpowder, such that an electrical connection with the semiconductorstructure is formed.

The ability of the paste composition to penetrate or etch through theanti-reflective layer and form a strong adhesive bond with the substrateupon firing is highly dependent on the composition of the conductivepaste and the firing conditions. Key measures of photovoltaic cellelectrical performance, such as efficiency, are also influenced by thequality of the electrical contact made between the fired conductivepaste and the substrate.

Although various methods and compositions useful in forming devices suchas photovoltaic cells are known, there nevertheless remains a need forcompositions that permit fabrication of patterned conductive structuresthat provide improved overall device electrical performance and thatfacilitate the efficient manufacture of such devices.

SUMMARY OF THE INVENTION

An embodiment of the invention relates to a paste compositioncomprising:

(a) a source of electrically conductive metal;

(b) an alkaline-earth-metal boron bismuth oxide; and

(c) an organic vehicle, in which the source of electrically conductivemetal and the oxide are dispersed.

In certain embodiments, the alkaline-earth-metal boron bismuth oxidefurther comprises an oxide of an additional cation, including, withoutlimitation, an oxide of any one or more of Al, Li, Na, K, Rb, Cs, Ti, V,Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Si, Mo, W, Hf, Ag, Ga, Ge, In, Sn,Sb, Se, Ru, P, Y, La and the other lanthanide elements, and mixturesthereof, or substances that form such oxides during heating. Otherembodiments of the paste composition further comprise one or morediscrete oxide additives, including, without limitation, oxides of Al,Li, Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Si, Mo, W,Hf, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, Bi, Ba, Ca, Sr, Mg, B, P, Y, La orthe other lanthanide elements, or mixtures thereof, or a compound of oneor more of the above elements which form an oxide upon firing.

Another aspect provides a process for forming an electrically conductivestructure on a substrate, the process comprising:

-   -   (a) providing a substrate having a first major surface;    -   (b) applying a paste composition onto a preselected portion of        the first major surface, wherein the paste composition comprises        in admixture:        -   i) a source of electrically conductive metal,        -   ii) an alkaline-earth-metal boron bismuth oxide, and        -   iii) an organic vehicle, in which the source of electrically            conductive metal and the oxide are dispersed; and    -   (c) firing the substrate and paste composition thereon, whereby        the electrically conductive structure is formed on the        substrate.

In a further implementation, the substrate includes an anti-reflectivelayer on its surface, and the firing results in the paste at leastpartially etching through the anti-reflective layer, such thatelectrical contact between the conductive structure and the substrate isestablished.

Further, there is provided an article comprising a substrate and anelectrically conductive structure thereon, the article having beenformed by the foregoing process. Representative articles of this typeinclude a semiconductor device and a photovoltaic cell. In anembodiment, the substrate comprises a silicon wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the preferred embodiments of the invention and theaccompanying drawings, wherein like reference numerals denote similarelements throughout the several views and in which:

FIGS. 1A-1F depict successive steps of a process by which asemiconductor device may be fabricated. The device in turn may beincorporated into a photovoltaic cell. Reference numerals as used inFIGS. 1A-1F include the following:

-   -   10: p-type substrate    -   12: first major surface (front side) of substrate 10    -   14: second major surface (back side) of substrate 10    -   20: n-type diffusion layer    -   30: insulating layer    -   40: p+ layer    -   60: aluminum paste formed on back side    -   61: aluminum back electrode (obtained by firing back-side        aluminum paste)    -   70: silver or silver/aluminum paste formed on back side    -   71: silver or silver/aluminum back electrode (obtained by firing        back-side paste)    -   500: conductive paste formed on front side according to the        invention    -   501: conductive front electrode according to the invention        (formed by firing front-side conductive paste)

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses the need for a process to manufacturehigh performance semiconductor devices having mechanically robust, highconductivity electrodes. The conductive paste composition providedherein is beneficially employed in the fabrication of front-sideelectrodes of photovoltaic devices. Ideally, a paste compositionpromotes the formation of a front-side metallization that: (a) adheresstrongly to the underlying semiconductor substrate; and (b) provides arelatively low resistance contact with the substrate. Suitable pastecompositions are believed to aid in etching surface insulating layersoften employed in semiconductor structures such as photovoltaic cells toallow contact between the conductive electrodes and the underlyingsemiconductor.

In an aspect, this invention provides a paste composition thatcomprises: a functional conductive component, such as a source ofelectrically conductive metal; an alkaline-earth-metal boron bismuthoxide; an optional discrete inorganic additive; and an organic vehicle.Certain embodiments involve a photovoltaic cell that includes aconductive structure made with the present paste composition. Such cellsmay provide any combination of one or more of high photovoltaicconversion efficiency, high fill factor, and low series resistance.

In various embodiments, the present paste composition may comprise, inadmixture, an inorganic solids portion comprising (a) about 75% to about99.5% by weight, or about 90 to about 99% by weight, or about 95 toabout 99% by weight, of a source of an electrically conductive metal;(b) about 0.5% to about 15% by weight, or about 0.5% to about 8% byweight, or about 2% to about 8% by weight, or about 0.5 to about 5% byweight, or about 1 to about 3% by weight, of an alkaline-earth-metalboron bismuth oxide material, wherein the above stated contents ofconstituents (a) and (b) are based on the total weight of all theconstituents of the inorganic solids portion of the composition, apartfrom the organic medium.

As further described below, the paste composition further comprises anorganic vehicle, which acts as a carrier for the inorganic constituents,which are dispersed therein. The paste composition may include stilladditional components such as surfactants, thickeners, thixotropes, andbinders.

Typically, electrodes and other conductive traces are provided by screenprinting the paste composition onto a substrate, although other forms ofprinting, such as plating, extrusion, inkjet, shaped or multipleprinting, or ribbons may also be used. After deposition, thecomposition, which typically comprises a conductive metal powder (e.g.,Ag), a frit, and optional inorganic additives in an organic carrier, isfired at an elevated temperature.

The composition also can be used to form conductive traces, such asthose employed in a semiconductor module that is to be incorporated intoan electrical or electronic device. As would be recognized by a skilledartisan, the paste composition described herein can be termed“conductive,” meaning that the composition can be formed into astructure and thereafter processed to exhibit an electrical conductivitysufficient for conducting electrical current between devices orcircuitry connected thereto.

I. Inorganic Components

An embodiment of the present invention relates to a paste composition,which may include: an inorganic solids portion comprising a functionalmaterial providing electrical conductivity, an alkaline-earth-metalboron bismuth oxide fusible material; and an organic vehicle in whichthe inorganic solids are dispersed. The paste composition may furtherinclude additional components such as surfactants, thickeners,thixotropes, and binders.

A. Electrically Conductive Metal

The present paste composition includes a source of an electricallyconductive metal. Exemplary metals include without limitation silver,gold, copper, nickel, palladium, platinum, aluminum, and alloys andmixtures thereof. Silver is preferred for its processability and highconductivity. However, a composition including at least somenon-precious metal may be used to reduce cost.

The conductive metal may be incorporated directly in the present pastecomposition as a metal powder. In another embodiment, a mixture of twoor more such metals is directly incorporated. Alternatively, the metalis supplied by a metal oxide or salt that decomposes upon exposure tothe heat of firing to form the metal. As used herein, the term “silver”is to be understood as referring to elemental silver metal, alloys ofsilver, and mixtures thereof, and may further include silver derivedfrom silver oxide (Ag₂O or AgO) or silver salts such as AgCl, AgNO₃,AgOOCCH₃ (silver acetate), AgOOCF₃ (silver trifluoroacetate), Ag₃PO₄(silver orthophosphate), or mixtures thereof. Any other form ofconductive metal compatible with the other components of the pastecomposition also may be used.

Electrically conductive metal powder used in the present pastecomposition may be supplied as finely divided particles having any oneor more of the following morphologies: a powder form, a flake form, aspherical form, a rod form, a granular form, a nodular form, acrystalline form, other irregular forms, or mixtures thereof. Theelectrically conductive metal or source thereof may also be provided ina colloidal suspension, in which case the colloidal carrier would not beincluded in any calculation of weight percentages of the solids of whichthe colloidal material is part.

The particle size of the metal is not subject to any particularlimitation. As used herein, “particle size” is intended to refer to“median particle size” or d₅₀, by which is meant the 50% volumedistribution size. The distribution may also be characterized by d₉₀,meaning that 90% by volume of the particles are smaller than d₉₀. Volumedistribution size may be determined by a number of methods understood byone of skill in the art, including but not limited to laser diffractionand dispersion methods employed by a Microtrac particle size analyzer(Montgomeryville, Pa.). Laser light scattering, e.g., using a modelLA-910 particle size analyzer available commercially from HoribaInstruments Inc. (Irvine, Calif.), may also be used. In variousembodiments, the median particle size is greater than 0.2 μm and lessthan 10 μm, or the median particle size is greater than 0.4 μm and lessthan 5 μm, as measured using the Horiba LA-910 analyzer.

The electrically conductive metal may comprise any of a variety ofpercentages of the composition of the paste composition. To attain highconductivity in a finished conductive structure, it is generallypreferable to have the concentration of the electrically conductivemetal be as high as possible while maintaining other requiredcharacteristics of the paste composition that relate to eitherprocessing or final use. In an embodiment, the silver or otherelectrically conductive metal may comprise about 75% to about 99.5% byweight, or about 85 to about 99.5% by weight, or about 95 to about 99%by weight, of the inorganic solid components of the paste composition.In another embodiment, the solids portion of the paste composition mayinclude about 80 to about 90 wt. % silver particles and about 1 to about9 wt. % silver flakes. In an embodiment, the solids portion of the pastecomposition may include about 70 to about 90 wt. % silver particles andabout 1 to about 9 wt. % silver flakes. In another embodiment, thesolids portion of the paste composition may include about 70 to about 90wt. % silver flakes and about 1 to about 9 wt. % of colloidal silver. Ina further embodiment, the solids portion of the paste composition mayinclude about 60 to about 90 wt. % of silver particles or silver flakesand about 0.1 to about 20 wt. % of colloidal silver.

The electrically conductive metal used herein, particularly when inpowder form, may be coated or uncoated; for example, it may be at leastpartially coated with a surfactant to facilitate processing. Suitablecoating surfactants include, for example, stearic acid, palmitic acid, asalt of stearate, a salt of palmitate, and mixtures thereof. Othersurfactants that also may be utilized include lauric acid, oleic acid,capric acid, myristic acid, linoleic acid, and mixtures thereof. Stillother surfactants that also may be utilized include polyethylene oxide,polyethylene glycol, benzotriazole, poly(ethylene glycol)acetic acid,and other similar organic molecules. Suitable counter-ions for use in acoating surfactant include without limitation hydrogen, ammonium,sodium, potassium, and mixtures thereof. When the electricallyconductive metal is silver, it may be coated, for example, with aphosphorus-containing compound.

In an embodiment, one or more surfactants may be included in the organicvehicle in addition to any surfactant included as a coating ofconductive metal powder used in the present paste composition.

As further described below, the electrically conductive metal can bedispersed in an organic vehicle that acts as a carrier for the metalphase and other constituents present in the formulation.

B. Alkaline-Earth-Metal Boron Bismuth Oxide

The present paste composition includes a fusible alkaline-earth-metalboron bismuth oxide. The term “fusible,” as used herein, refers to theability of a material to become fluid upon heating, such as the heatingemployed in a firing operation. In some embodiments, the fusiblematerial is composed of one or more fusible subcomponents. For example,the fusible material may comprise a glass material, or a mixture of twoor more glass materials. Glass material in the form of a fine powder,e.g., as the result of a comminution operation, is often termed “frit”and is readily incorporated in the present paste composition.

As used herein, the term “glass” refers to a particulate solid form,such as an oxide or oxyfluoride, that is at least predominantlyamorphous, meaning that short-range atomic order is preserved in theimmediate vicinity of any selected atom, that is, in the firstcoordination shell, but dissipates at greater atomic-level distances(i.e., there is no long-range periodic order). Hence, the X-raydiffraction pattern of a fully amorphous material exhibits broad,diffuse peaks, and not the well-defined, narrow peaks of a crystallinematerial. In the latter, the regular spacing of characteristiccrystallographic planes give rise to the narrow peaks, whose position inreciprocal space is in accordance with Bragg's law. A glass materialalso does not show a substantial crystallization exotherm upon heatingclose to or above its glass transition temperature or softening point,T_(g), which is defined as the second transition point seen in adifferential thermal analysis (DTA) scan. In an embodiment, thesoftening point of glass material used in the present paste compositionis in the range of 300 to 800° C.

It is also contemplated that some or all of the alkaline-earth-metalboron bismuth oxide material may be composed of material that exhibitssome degree of crystallinity. For example, in some embodiments, aplurality of oxides are melted together and quenched as set forth above,resulting in a material that is partially amorphous and partiallycrystalline. As would be recognized by a skilled person, such a materialwould produce an X-ray diffraction pattern having narrow, crystallinepeaks superimposed on a pattern with broad, diffuse peaks.Alternatively, one or more constituents, or even substantially all ofthe fusible material, may be predominantly or even substantially fullycrystalline. In an embodiment, crystalline material useful in thefusible material of the present paste composition may have a meltingpoint of at most 800° C.

The fusible material used in the present paste composition is analkaline-earth-metal boron bismuth oxide. As used herein, the term“alkaline-earth-metal boron bismuth oxide” refers to an oxide materialcontaining alkaline-earth metal, boron, and bismuth cations thattogether comprise at least 75% of the cations present in the material,and wherein the minimum content of alkaline-earth metal, boron, andbismuth cations is at least 10, 14, and 10 cation %, respectively. Invarious embodiments, the combination of alkaline-earth metal, boron, andbismuth cations represents at least 75%, 80%, 90%, 95%, or up to 100% ofthe cations in the alkaline-earth metal boron oxide. The alkaline-earthmetals useful in the present paste composition are Mg, Ca, Sr, Ba, andmixtures thereof.

The alkaline-earth-metal boron bismuth oxide used in the present pastecomposition is described herein as including percentages of certaincomponents. Specifically, the composition may be specified bydenominating individual components that may be combined in the specifiedpercentages to form a starting material that subsequently is processed,e.g., as described herein, to form a glass or other fusible material.Such nomenclature is conventional to one of skill in the art. In otherwords, the composition contains certain components, and the percentagesof those components may be expressed as weight percentages of thecorresponding oxide or other forms.

Alternatively, some of the compositions herein are set forth by cationpercentages, which are based on the total cations contained in thealkaline-earth-metal boron bismuth oxide. Of course, compositions thusspecified include the oxygen or other anions associated with the variouscations. A skilled person would recognize that compositions couldequivalently be specified by weight percentages of the constituents, andwould be able to perform the required numerical conversions. The skilledperson would also recognize that some of the cations incorporated in thepresent composition can exist in different valences. Compounds used toformulate the composition thus may have such cations in any convenientvalence.

The alkaline-earth-metal boron bismuth oxide included in the presentpaste composition optionally incorporates other oxides, including oxidesof one or more of the elements Al, Li, Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe,Co, Ni, Cu, Zn, Zr, Nb, Si, Mo, W, Hf, Ag, Ga, Ge, In, Sn, Sb, Se, Ru,P, Y, La and the other lanthanide elements, and mixtures thereof. (Theterm “lanthanide elements” is understood to include the chemicalelements of the periodic table having atomic numbers of 57 through 71,i.e., La-Lu.) This list is meant to be illustrative, not limiting. Inanother embodiment, the alkaline-earth-metal boron bismuth oxide furthercomprises an oxide of one or more of the elements Li, P, Ti, Zn, Si, orAg. The foregoing substances are intimately mixed at an atomic level inthe alkaline-earth-metal boron bismuth oxide, e.g., by melting thesubstances together. In some embodiments, the amount of these otheroxides incorporated is such that the total cation percentage of them inthe alkaline-earth-metal boron bismuth oxide is up to 5, 10, 20, or 25%.

Although oxygen is typically the predominant anion in thealkaline-earth-metal boron bismuth oxide of the present pastecomposition, some portion of the oxygen may be replaced by fluorine orother halogen anions to alter certain properties, such as chemical,thermal, or rheological properties of the oxide that affect firing. Inan embodiment, up to 10% of the oxygen anions of thealkaline-earth-metal boron bismuth oxide in any of the formulations ofthe present paste composition are replaced by one or more halogenanions, including fluorine. For example, up to 10% of the oxygen anionsmay be replaced by fluorine. Halogen anions may be supplied from halidesof any of the composition's cations, including, but not limited to,NaCl, KBr, NaI, LiF, CaF₂, MgF₂, BaCl₂, and BiF₃. In an embodiment, upto 10 anion percent of these oxygen anions can be substituted byhalogens, including fluorine. In another embodiment, the replacement ofoxygen may provide a content of up to 5 wt. % F, CI, or Br.

For example, one of ordinary skill would recognize that embodimentswherein the alkaline-earth-metal boron bismuth oxide contains fluorinecan be prepared using fluorine anions supplied from a simple fluoride oran oxyfluoride. In an embodiment, the desired fluorine content can besupplied by replacing some or all of an oxide nominally incorporated inthe composition with the corresponding fluoride of the same cation, suchas by replacing some or all of the MgO, CaO, SrO, or BaO nominallyincluded with the amount of MgF₂, CaF₂, SrF₂, or BaF₂ needed to attainthe desired level of F content. Of course, the requisite amount of F canbe derived by replacing the oxides of more than one cation of thealkaline-earth-metal boron bismuth oxide if desired. Other fluoridesources could also be used, including sources such as ammonium fluoridethat would decompose during the heating in typical glass preparation toleave behind residual fluoride anions. Useful fluorides include, but arenot limited to, CaF₂, BiF₃, AlF₃, NaF, LiF, ZrF₄, TiF₄, and ZnF₂.

The present paste composition may further comprise an optional discreteoxide additive. It is contemplated that the additive may comprise anoxide of one element, two or more discrete oxides of various elements,or a discrete mixed oxide of multiple elements. As used herein, the term“oxide of an element” includes both the oxide compound itself and anyother organic or inorganic compound of the element, or the pure elementitself if it oxidizes or decomposes on heating to form the pertinentoxide. Such compounds known to decompose upon heating include, but arenot limited to, carbonates, nitrates, nitrites, hydroxides, acetates,formates, citrates, and soaps of the foregoing elements, and mixturesthereof. For example, Zn metal, zinc acetate, zinc carbonate, and zincmethoxide are potential additives that would oxidize or decompose toform zinc oxide upon firing. The oxide additive is discrete, in that itis not mixed at an atomic level with the base alkaline-earth metal boronoxide, but is separately present in the paste composition. In anembodiment, the discrete oxide additive may be present in the pastecomposition in an amount ranging from 0.01 to 5 wt. %, or 0.05 to 2.5wt. %, or 0.1 to 1 wt. %, based on the total weight of the pastecomposition.

Although in some embodiments the present composition (including thefusible material contained therein) may contain a small amount of lead,lead oxide, or other lead compound, e.g., in an amount up to 5 cation %in the alkaline-earth-metal boron bismuth oxide, other embodiments arelead-free. As used herein, the term “lead-free paste composition” refersto a paste composition to which no lead has been specifically added(either as elemental lead or as a lead-containing alloy, compound, orother like substance), and in which the amount of lead present as atrace component or impurity is 1000 parts per million (ppm) or less. Insome embodiments, the amount of lead present as a trace component orimpurity is less than 500 ppm, or less than 300 ppm, or less than 100ppm. Surprisingly and unexpectedly, photovoltaic cells exhibitingdesirable electrical properties, such as high conversion efficiency, areobtained in some embodiments of the present disclosure, notwithstandingprevious belief in the art that substantial amounts of lead must beincluded in a paste composition to attain these levels.

Similarly, some embodiments of the present paste composition comprisecadmium, e.g., in an amount up to 5 cation % in the alkaline-earth-metalboron bismuth oxide, while others are cadmium-free, again meaning thatno Cd metal or compound is specifically added and that the amountpresent as a trace impurity is less than 1000 ppm, 500 ppm, 300 ppm, or100 ppm.

In various embodiments, the alkaline-earth-metal boron bismuth oxide ofthe present paste composition comprises, or consists essentially of:

10 to 40, or 10 to 35, or 12 to 30 cation % of an alkaline earth metalselected from the group of Mg, Ca, Ba, Sr, and mixtures thereof;

14 to 65, or 25 to 65, or 30 to 60 cation % of B; and

10 to 60, or 12 to 55, or 15 to 50 cation % of Bi.

In other embodiments, the alkaline-earth-metal boron bismuth oxide ofthe present paste composition comprises, or consists essentially of:

10 to 40, or 10 to 35, or 12 to 30 cation % of alkaline earth metalselected from the group of Mg, Ca, Ba, Sr, and mixtures thereof;

14 to 65, or 25 to 65, or 30 to 60 cation % of B;

10 to 60, or 12 to 55, or 15 to 50 cation % of Bi;

0 to 15, or 0 to 10, or 0 to 5 cation % of Li;

0 to 15, or 0 to 10, or 0 to 5 cation % of Na;

0 to 15, or 0 to 10, or 0 to 7 cation % of Si;

0 to 15, or 0 to 10, or 0 to 8 cation % of P;

0 to 20, or 0 to 15, or 0 to 10 cation % of Zn; and

0 to 20, or 0 to 10, or 0 to 5 cation % of Ti,

plus incidental impurities.

One of ordinary skill in the art of glass chemistry would furtherrecognize that any of the foregoing alkaline-earth-metal boron bismuthoxide material compositions, whether specified by weight percentages orcation percentages of its constituent oxides, may alternatively beprepared by supplying the required anions and cations in requisiteamounts from different components that, when mixed and fired, yield thesame overall composition. For example, in various embodiments,phosphorus cations could be supplied either from P₂O₅, or alternativelyfrom a suitable organic or inorganic phosphate that decomposes onheating to yield P₂O₅, or from a metal phosphate in which the metal isalso a desired component of the final material. Lithium cations could besupplied from Li₂O, or alternatively from a suitable organic orinorganic compound that decomposes on heating to yield Li₂O, such aslithium carbonate, lithium acetate, or lithium hydroxide, or from amixed metal oxide that includes lithium, such as a lithium titaniumoxide. It will be understood that the term “mixed metal oxide” refers toan oxide comprising two or more cations. Such mixed metal oxides includeones in which the cations are located randomly or in defined positionsof a crystallographic structure. The skilled person would also recognizethat depending on the starting material employed, a certain portion ofvolatile species, e.g., carbon dioxide, may be released during theprocess of making a fusible material.

It is known to those skilled in the art that an alkaline-earth-metalboron bismuth oxide such as one prepared by a melting technique asdescribed herein may be characterized by known analytical methods thatinclude, but are not limited to: Inductively Coupled Plasma-EmissionSpectroscopy (ICP-ES), Inductively Coupled Plasma-Atomic EmissionSpectroscopy (ICP-AES), and the like. In addition, the followingexemplary techniques may be used: X-Ray Fluorescence spectroscopy (XRF),Nuclear Magnetic Resonance spectroscopy (NMR), Electron ParamagneticResonance spectroscopy (EPR), Mössbauer spectroscopy, electronmicroprobe Energy Dispersive Spectroscopy (EDS), electron microprobeWavelength Dispersive Spectroscopy (WDS), and Cathodoluminescence (CL).A skilled person could calculate percentages of starting components thatcould be processed to yield a particular fusible material, based onresults obtained with such analytical methods.

The embodiments of the alkaline-earth-metal boron bismuth oxide materialdescribed herein, including the compositions listed in Tables I and IV,are not limiting; it is contemplated that one of ordinary skill in theart of glass chemistry could make minor substitutions of additionalingredients and not substantially change the desired properties of thealkaline-earth-metal boron bismuth oxide composition, including itsinteraction with a substrate and any insulating layer thereon.

A median particle size of the alkaline-earth-metal boron bismuth oxidematerial in the present composition may be in the range of about 0.5 to10 μm, or about 0.8 to 5 μm, or about 1 to 3 μm, as measured using theHoriba LA-910 analyzer.

In an embodiment, the alkaline-earth-metal boron bismuth oxide may beproduced by conventional glass-making techniques and equipment. For theexamples provided herein, the ingredients were weighed and mixed in thedesired proportions and heated in a platinum alloy crucible in afurnace. The ingredients may be heated to a peak temperature (e.g., 800°C. to 1400° C., or 1000° C. to 1200° C.) and held for a time such thatthe material forms a melt that is substantially liquid and homogeneous(e.g., 20 minutes to 2 hours). The melt optionally is stirred, eitherintermittently or continuously. In an embodiment, the melting processresults in a material wherein the constituent chemical elements arefully mixed at an atomic level. The molten material is then typicallyquenched in any suitable way including, without limitation, passing itbetween counter-rotating stainless steel rollers to form 0.25 to 0.50 mmthick platelets, by pouring it onto a thick stainless steel plate, or bypouring it into water or other quench fluid. The resulting particles arethen milled to form a powder or frit, which typically may have a d₅₀ of0.2 to 3.0 μm.

Other production techniques may also be used for the presentalkaline-earth-metal boron bismuth oxide material. One skilled in theart of producing such materials might therefore employ alternativesynthesis techniques including, but not limited to, melting innon-precious metal crucibles, melting in ceramic crucibles, sol-gel,spray pyrolysis, or others appropriate for making powder forms of glass.

A skilled person would recognize that the choice of raw materials couldunintentionally include impurities that may be incorporated into thealkaline-earth-metal boron bismuth oxide material during processing. Forexample, these incidental impurities may be present in the range ofhundreds to thousands of parts per million. Impurities commonlyoccurring in industrial materials used herein are known to one ofordinary skill.

The presence of the impurities would not substantially alter theproperties of the alkaline-earth-metal boron bismuth oxide itself, pastecompositions made with the alkaline-earth-metal boron bismuth oxide, ora fired device manufactured using the paste composition. For example, asolar cell employing a conductive structure made using the present pastecomposition may have the efficiency described herein, even if thecomposition includes impurities.

The alkaline-earth-metal boron bismuth oxide used in the presentcomposition is believed to assist in the partial or complete penetrationof the oxide or nitride insulating layer commonly present on a siliconsemiconductor wafer during firing. As described herein, this at leastpartial penetration may facilitate the formation of an effective,mechanically robust electrical contact between a conductive structuremanufactured using the present composition and the underlying siliconsemiconductor of a photovoltaic device structure.

The alkaline-earth-metal boron bismuth oxide material in the presentpaste composition may optionally comprise a plurality of separatefusible substances, such as one or more frits, or a substantiallycrystalline material with additional frit material. In an embodiment, afirst fusible subcomponent is chosen for its capability to rapidly etchan insulating layer, such as that typically present on the front surfaceof a photovoltaic cell; further the first fusible subcomponent may havestrong etching power and low viscosity. A second fusible subcomponent isoptionally included to slowly blend with the first fusible subcomponentto alter the chemical activity. Preferably, the composition is such thatthe insulating layer is partially removed but without attacking theunderlying emitter diffused region, which would shunt the device, werethe corrosive action to proceed unchecked. Such fusible materials may becharacterized as having a viscosity sufficiently high to provide astable manufacturing window to remove insulating layers without damageto the diffused p-n junction region of a semiconductor substrate.Ideally, the firing process results in a substantially complete removalof the insulating layer without further combination with the underlyingSi substrate or the formation of substantial amounts of non-conductingor poorly conducting inclusions.

C. Optional Oxide Additive

As noted above, an optional oxide may be included in the present pastecomposition as a discrete additive, such as an oxide of one or more ofAl, Li, Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Si,Mo, W, Hf, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, Bi, P, Y, La, or mixturesthereof, or a substance that forms such an oxide upon heating. The oxideadditive can be incorporated in the paste composition in a powder formas received from the supplier, or the powder can be ground or milled toa smaller average particle size. Particles of any size can be employed,as long as they can be incorporated into the present paste compositionand provide its required functionality. In an embodiment, the pastecomposition comprises up to 5 wt. % of the discrete oxide additive.

Any size-reduction method known to those skilled in the art can beemployed to reduce particle size to a desired level. Such processesinclude, without limitation, ball milling, media milling, jet milling,vibratory milling, and the like, with or without a solvent present. If asolvent is used, water is the preferred solvent, but other solvents maybe employed as well, such as alcohols, ketones, and aromatics.Surfactants may be added to the solvent to aid in the dispersion of theparticles, if desired.

II. Organic Vehicle

The inorganic components of the present composition are typically mixedwith an organic vehicle to form a relatively viscous material referredto as a “paste” or an “ink” that has a consistency and rheology thatrender it suitable for printing processes, including without limitationscreen printing. The mixing is typically done with a mechanical system,and the constituents may be combined in any order, as long as they areuniformly dispersed and the final formulation has characteristics suchthat it can be successfully applied during end use.

A wide variety of inert materials can be admixed in an organic medium inthe present composition including, without limitation, an inert,non-aqueous liquid that may or may not contain thickeners, binders, orstabilizers. By “inert” is meant a material that may be removed by afiring operation without leaving any substantial residue and that has noother effects detrimental to the paste or the final conductor lineproperties.

The proportions of organic vehicle and inorganic components in thepresent paste composition can vary in accordance with the method ofapplying the paste and the kind of organic vehicle used. In anembodiment, the present paste composition typically contains about 50 to95 wt. %, 76 to 95 wt. %, or 85 to 95 wt. %, of the inorganic componentsand about 5 to 50 wt. %, 5 to 24 wt. %, or 5 to 15 wt. %, of the organicvehicle.

The organic vehicle typically provides a medium in which the inorganiccomponents are dispersible with a good degree of stability. Inparticular, the composition preferably has a stability compatible notonly with the requisite manufacturing, shipping, and storage, but alsowith conditions encountered during deposition, e.g., by a screenprinting process. Ideally, the rheological properties of the vehicle aresuch that it lends good application properties to the composition,including stable and uniform dispersion of solids, appropriate viscosityand thixotropy for printing, appropriate wettability of the paste solidsand the substrate on which printing will occur, a rapid drying rateafter deposition, and stable firing properties.

Substances useful in the formulation of the organic vehicle of thepresent paste composition include, without limitation, ones disclosed inU.S. Pat. No. 7,494,607 and International Patent Application PublicationNo. WO 2010/123967 A2, both of which are incorporated herein in theirentirety for all purposes, by reference thereto. The disclosedsubstances include ethylhydroxyethyl cellulose, wood rosin, mixtures ofethyl cellulose and phenolic resins, cellulose acetate, celluloseacetate butyrate, polymethacrylates of lower alcohols, monobutyl etherof ethylene glycol, monoacetate ester alcohols, and terpenes such asalpha- or beta-terpineol or mixtures thereof with other solvents such askerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate,hexylene glycol, and high-boiling alcohols and alcohol esters.

Solvents useful in the organic vehicle include, without limitation,ester alcohols and terpenes such as alpha- or beta-terpineol or mixturesthereof with other solvents such as kerosene, dibutylphthalate, butylcarbitol, butyl carbitol acetate, hexylene glycol, and high-boilingalcohols and alcohol esters. A preferred ester alcohol is themonoisobutyrate of 2,2,4-trimethyl-1,3-pentanediol, which is availablecommercially from Eastman Chemical (Kingsport, Tenn.) as TEXANOL™. Someembodiments may also incorporate volatile liquids in the organic vehicleto promote rapid hardening after application on the substrate. Variouscombinations of these and other solvents are formulated to provide thedesired viscosity and volatility.

In an embodiment, the organic vehicle may include one or more componentsselected from the group consisting of: bis(2-(2butoxyethoxy)ethyl)adipate, dibasic esters, octyl epoxy tallate, isotetradecanol, and apentaerythritol ester of hydrogenated rosin. The paste compositions mayalso include additional additives or components.

The dibasic ester useful in the present paste composition may compriseone or more dimethyl esters selected from the group consisting ofdimethyl ester of adipic acid, dimethyl ester of glutaric acid, anddimethyl ester of succinic acid. Various forms of such materialscontaining different proportions of the dimethyl esters are availableunder the DBE® trade name from Invista (Wilmington, Del.). For thepresent paste composition, a preferred version is sold as DBE-3 and issaid by the manufacturer to contain 85 to 95 weight percent dimethyladipate, 5 to 15 weight percent dimethyl glutarate, and 0 to 1.0 weightpercent dimethyl succinate based on total weight of dibasic ester.

Further ingredients optionally may be incorporated in the organicvehicle, such as thickeners, stabilizers, and/or other common additivesknown to those skilled in the art. The organic vehicle may be a solutionof one or more polymers in a solvent. Additionally, effective amounts ofadditives, such as surfactants or wetting agents, may be a part of theorganic vehicle. Such added surfactant may be included in the organicvehicle in addition to any surfactant included as a coating on theconductive metal powder of the paste composition. Suitable wettingagents include phosphate esters and soya lecithin. Both inorganic andorganic thixotropes may also be present.

Among the commonly used organic thixotropic agents are hydrogenatedcastor oil and derivatives thereof, but other suitable agents may beused instead of, or in addition to, these substances. It is, of course,not always necessary to incorporate a thixotropic agent since thesolvent and resin properties coupled with the shear thinning inherent inany suspension may alone be suitable in this regard.

A polymer frequently used in printable conductive metal pastes is ethylcellulose. Other exemplary polymers that may be used includeethylhydroxyethyl cellulose, wood rosin and derivatives thereof,mixtures of ethyl cellulose and phenolic resins, cellulose acetate,cellulose acetate butyrate, poly(methacrylate)s of lower alcohols, andmonoalkyl ethers of ethylene glycol monoacetate.

Any of these polymers may be dissolved in a suitable solvent, includingthose described herein.

The polymer in the organic vehicle may be present in the range of 0.1wt. % to 5 wt. % of the total composition. The present paste compositionmay be adjusted to a predetermined, screen-printable viscosity, e.g.,with additional solvent(s).

III. Formation of Conductive Structures

An aspect of the invention provides a process that may be used to form aconductive structure on a substrate. The process generally comprises thesteps of providing the substrate, applying a paste composition, andfiring the substrate. Ordinarily, the substrate is planar and relativelythin, thus defining first and second major surfaces on its oppositesides.

Application

The present composition can be applied as a paste onto a preselectedportion of a major surface of the substrate in a variety of differentconfigurations or patterns. The preselected portion may comprise anyfraction of the total first major surface area, including substantiallyall of the area. In an embodiment, the paste is applied on asemiconductor substrate, which may be single-crystal, cast mono,multi-crystal, polycrystalline, or ribbon silicon, or any othersemiconductor material.

The application can be accomplished by a variety of depositionprocesses, including printing. Exemplary deposition processes include,without limitation, plating, extrusion or co-extrusion, dispensing froma syringe, and screen, inkjet, shaped, multiple, and ribbon printing.The paste composition ordinarily is applied over any insulating layerpresent on the first major surface of the substrate.

The conductive composition may be printed in any useful pattern. Forexample, the electrode pattern used for the front side of a photovoltaiccell commonly includes a plurality of narrow grid lines or fingersconnected to one or more bus bars. In an embodiment, the width of thelines of the conductive fingers may be 20 to 200 μm; 25 to 100 μm; or 35to 75 μm. In an embodiment, the thickness of the lines of the conductivefingers may be 5 to 50 μm; 10 to 35 μm; or 15 to 30 μm. Such a patternpermits the generated current to be extracted without undue resistiveloss, while minimizing the area of the front side obscured by themetallization, which reduces the amount of incoming light energy thatcan be converted to electrical energy. Ideally, the features of theelectrode pattern should be well defined, with a preselected thicknessand shape, and have high electrical conductivity and low contactresistance with the underlying structure.

Conductors formed by printing and firing a paste such as that providedherein are often denominated as “thick film” conductors, since they areordinarily substantially thicker than traces formed by atomisticprocesses, such as those used in fabricating integrated circuits. Forexample, thick film conductors may have a thickness after firing ofabout 1 to 100 μm. Consequently, paste compositions that in theirprocessed form provide conductivity and are suitably applied usingprinting processes are often called “thick film pastes” or “conductiveinks.”

Firing

A firing operation may be used in the present process to effect asubstantially complete burnout of the organic vehicle from the depositedpaste. The firing typically involves volatilization and/or pyrolysis ofthe organic materials. A drying operation optionally precedes the firingoperation, and is carried out at a modest temperature to harden thepaste composition by removing its most volatile organics.

The firing process is believed to remove the organic vehicle, sinter theconductive metal in the composition, and establish electrical contactbetween the semiconductor substrate and the fired conductive metal.Firing may be performed in an atmosphere composed of air, nitrogen, aninert gas, or an oxygen-containing mixture such as a mixed gas of oxygenand nitrogen.

In one embodiment, the temperature for the firing may be in the rangebetween about 300° C. to about 1000° C., or about 300° C. to about 525°C., or about 300° C. to about 650° C., or about 650° C. to about 1000°C. The firing may be conducted using any suitable heat source. In anembodiment, the firing is accomplished by passing the substrate bearingthe printed paste composition pattern through a belt furnace at hightransport rates, for example between about 100 to about 500 cm perminute, with resulting hold-up times between about 0.05 to about 5minutes. Multiple temperature zones may be used to control the desiredthermal profile, and the number of zones may vary, for example, between3 to 11 zones. The temperature of a firing operation conducted using abelt furnace is conventionally specified by the furnace set point in thehottest zone of the furnace, but it is known that the peak temperatureattained by the passing substrate in such a process is somewhat lowerthan the highest set point. Other batch and continuous rapid firefurnace designs known to one of skill in the art are also contemplated.

In a further embodiment, other conductive and device enhancing materialsare applied prior to firing to the opposite type region of thesemiconductor device. The various materials may be applied and thenco-fired, or they may be applied and fired sequentially.

In an embodiment, the opposite type region may be on the non-illuminated(back) side of the device, i.e., its second major surface. The materialsserve as electrical contacts, passivating layers, and solderable tabbingareas. In an aspect of this embodiment, the back-side conductivematerial may contain aluminum. Exemplary back-side aluminum-containingcompositions and methods of application are described, for example, inUS 2006/0272700, which is hereby incorporated herein in its entirety forall purposes by reference thereto. Suitable solderable tabbing materialsinclude those containing aluminum and silver. Exemplary tabbingcompositions containing aluminum and silver are described, for examplein US 2006/0231803, which is hereby incorporated herein in its entiretyfor all purposes by reference thereto.

In a further embodiment, the present paste composition may be employedin the construction of semiconductor devices wherein the p and n regionsare formed side-by-side in a substrate, instead of being respectivelyadjacent to opposite major surfaces of the substrate. In animplementation in this configuration, the electrode-forming materialsmay be applied in different portions of a single side of the substrate,e.g., on the non-illuminated (back) side of the device, therebymaximizing the amount of light incident on the illuminated (front) side.

Insulating Layer

In some embodiments of the invention, the paste composition is used inconjunction with a substrate, such as a semiconductor substrate, havingan insulating layer present on one or more of the substrate's majorsurfaces. The layer may comprise one or more components selected fromaluminum oxide, titanium oxide, silicon nitride, SiN_(x):H (siliconnitride containing hydrogen for passivation during subsequent firingprocessing), silicon oxide, and silicon oxide/titanium oxide, and may bein the form of a single, homogeneous layer or multiple sequentialsub-layers of any of these materials. Silicon nitride and SiN_(x):H arewidely used.

The insulating layer provides some embodiments of the cell with ananti-reflective property, which lowers the cell's surface reflectance oflight incident thereon, thereby improving the cell's utilization of theincident light and increasing the electrical current it can generate.Thus, the insulating layer is often denoted as an anti-reflectivecoating (ARC). The thickness of the layer preferably is chosen tomaximize the anti-reflective property in accordance with the layermaterial's composition and refractive index. In one approach, thedeposition processing conditions are adjusted to vary the stoichiometryof the layer, thereby altering properties such as the refractive indexto a desired value. For a silicon nitride layer with a refractive indexof about 1.9 to 2.0, a thickness of about 700 to 900 Å (70 to 90 nm) issuitable.

The insulating layer may be deposited on the substrate by methods knownin the microelectronics art, such as any form of chemical vapordeposition (CVD) including plasma-enhanced CVD (PECVD) and thermal CVD,thermal oxidation, or sputtering. In another embodiment, the substrateis coated with a liquid material that under thermal treatment decomposesor reacts with the substrate to form the insulating layer. In stillanother embodiment, the substrate is thermally treated in the presenceof an oxygen- or nitrogen-containing atmosphere to form an insulatinglayer. Alternatively, no insulating layer is specifically applied to thesubstrate, but a naturally forming substance, such as silicon oxide on asilicon wafer, may function as an insulating layer.

The present method optionally includes the step of forming theinsulating layer on the semiconductor substrate prior to the applicationof the paste composition.

In some implementations of the present process, the paste composition isapplied over any insulating layer present on the substrate, whetherspecifically applied or naturally occurring. The paste's fusiblematerial and any additive present may act in concert to combine with,dissolve, or otherwise penetrate some or all of the thickness of anyinsulating layer material during firing. Preferably, good electricalcontact between the paste composition and the underlying semiconductorsubstrate is thereby established. Ideally, the firing results in asecure attachment of the conductive metal structure to the substrate,with a metallurgical bond being formed over substantially all the areaof the substrate covered by the conductive element. In an embodiment,the conductive metal is separated from the silicon by a nanometer-scaleinterfacial film layer (typically of order 5 nm or less) through whichthe photoelectrons tunnel. In another embodiment, contact is madebetween the conductive metal and the silicon by a combination of directmetal-to-silicon contact and tunneling through thin interfacial filmlayers.

Firing also promotes the formation of both good electrical conductivityin the conductive element itself and a low-resistance connection to thesubstrate, e.g., by sintering the conductive metal particles and etchingthrough the insulating layer. While some embodiments may function withelectrical contact that is limited to conductive domains dispersed overthe printed area, it is preferred that the contact be uniform oversubstantially the entire printed area.

Structures

An embodiment of the present invention relates to a structure comprisinga substrate and a conductive electrode, which may be formed by theprocess described above.

Semiconductor Device Manufacture

The structures described herein may be useful in the manufacture ofsemiconductor devices, including photovoltaic devices. An embodiment ofthe invention relates to a semiconductor device containing one or morestructures described herein. Another embodiment relates to aphotovoltaic device containing one or more structures described herein.Still further, there is provided a photovoltaic cell containing one ormore structures described herein and a solar panel containing one ormore of these structures.

In another aspect, the present invention relates to a device, such as anelectrical, electronic, semiconductor, photodiode, or photovoltaicdevice. Various embodiments of the device include a junction-bearingsemiconductor substrate and an insulating layer, such as a siliconnitride layer, present on a first major surface of the substrate.

One possible sequence of steps implementing the present process formanufacture of a photovoltaic cell device is depicted by FIGS. 1A-1F.

FIG. 1A shows a p-type substrate 10, which may be single-crystal,multi-crystalline, or polycrystalline silicon. For example, substrate 10may be obtained by slicing a thin layer from an ingot that has beenformed from a pulling or casting process. Surface damage andcontamination (from slicing with a wire saw, for example) may be removedby etching away about 10 to 20 μm of the substrate surface using anaqueous alkali solution such as aqueous potassium hydroxide or aqueoussodium hydroxide, or using a mixture of hydrofluoric acid and nitricacid. In addition, the substrate may be washed with a mixture ofhydrochloric acid and optional hydrogen peroxide to remove heavy metalssuch as iron adhering to the substrate surface. Substrate 10 may have afirst major surface 12 that is textured to reduce light reflection.Texturing may be produced by etching a major surface with an aqueousalkali solution such as aqueous potassium hydroxide or aqueous sodiumhydroxide. Substrate 10 may also be formed from a silicon ribbon.

In FIG. 1B, an n-type diffusion layer 20 is formed to create a p-njunction with p-type material below. The n-type diffusion layer 20 canbe formed by any suitable doping process, such as thermal diffusion ofphosphorus (P) provided from phosphorus oxychloride (POCl₃) or ionimplantation. In the absence of any particular modifications, the n-typediffusion layer 20 is formed over the entire surface of the siliconp-type substrate. The depth of the diffusion layer can be varied bycontrolling the diffusion temperature and time, and is generally formedin a thickness range of about 0.3 to 0.5 μm. The n-type diffusion layermay have a sheet resistivity from several tens of ohms per square up toabout 120 ohms per square.

After protecting one surface of the n-type diffusion layer 20 with aresist or the like, the n-type diffusion layer 20 is removed from mostsurfaces by etching so that it remains only on the first major surface12 of substrate 10, as shown in FIG. 1C. The resist is then removedusing an organic solvent or the like.

Next, as shown in FIG. 1D, an insulating layer 30, which also functionsas an anti-reflective coating, is formed on the n-type diffusion layer20. The insulating layer is commonly silicon nitride, but can also be alayer of another material, such as SiN_(x):H (i.e., the insulating layercomprises hydrogen for passivation during subsequent firing processing),titanium oxide, silicon oxide, mixed silicon oxide/titanium oxide, oraluminum oxide. The insulating layer can be in the form of a singlelayer or multiple layers of the same or different materials.

Next, electrodes are formed on both major surfaces 12 and 14 of thesubstrate. As shown in FIG. 1E, a paste composition 500 of thisinvention is screen printed on the insulating layer 30 of the firstmajor surface 12 and then dried. For a photovoltaic cell, pastecomposition 500 is typically applied in a predetermined pattern ofconductive lines extending from one or more bus bars that occupy apredetermined portion of the surface. In addition, aluminum paste 60 andback-side silver paste 70 are screen printed onto the back side (thesecond major surface 14 of the substrate) and successively dried. Thescreen printing operations may be carried out in any order. For the sakeof production efficiency, all these pastes are typically processed byco-firing them at a temperature in the range of about 700° C. to about975° C. for a period of from several seconds to several tens of minutesin air or an oxygen-containing atmosphere. An infrared-heated beltfurnace is conveniently used for high throughput.

As shown in FIG. 1F, the firing causes the depicted paste composition500 on the front side to sinter and penetrate through the insulatinglayer 30, thereby achieving electrical contact with the n-type diffusionlayer 20, a condition known as “fire through.” This fired-through state,i.e., the extent to which the paste reacts with and passes through theinsulating layer 30, depends on the quality and thickness of theinsulating layer 30, the composition of the paste, and on the firingconditions. A high-quality fired-through state is believed to be animportant factor in obtaining high conversion efficiency in aphotovoltaic cell. Firing thus converts paste 500 into electrode 501, asshown in FIG. 1F.

The firing further causes aluminum to diffuse from the back-sidealuminum paste into the silicon substrate, thereby forming a p+ layer40, containing a high concentration of aluminum dopant. This layer isgenerally called the back surface field (BSF) layer, and helps toimprove the energy conversion efficiency of the solar cell. Firingconverts the dried aluminum paste 60 to an aluminum back electrode 61.The back-side silver paste 70 is fired at the same time, becoming asilver or silver/aluminum back electrode 71. During firing, the boundarybetween the back-side aluminum and the back-side silver assumes thestate of an alloy, thereby achieving electrical connection. Most areasof the back electrode are occupied by the aluminum electrode, owing inpart to the need to form a p+ layer 40. Since there is no need forincoming light to penetrate the back side, substantially the entiresurface may be covered. At the same time, because soldering to analuminum electrode is unfeasible, a silver or silver/aluminum backelectrode is formed on limited areas of the back side as an electrode topermit soldered attachment of interconnecting copper ribbons or thelike.

While the present invention is not limited by any particular theory ofoperation, it is believed that, upon firing, the alkaline-earth-metalboron bismuth oxide material, with any additive component present actingin concert, promotes rapid etching of the insulating layerconventionally used on the front side of a photovoltaic cell. Efficientetching in turn permits the formation of a low resistance, front-sideelectrical contact between the metal(s) of the composition and theunderlying substrate.

It will be understood that the present paste composition and process mayalso be used to form electrodes, including a front-side electrode, of aphotovoltaic cell in which the p- and n-type layers are reversed fromthe construction shown in FIGS. 1A-1F, so that the substrate is n-typeand a p-type material is formed on the front side.

In yet another embodiment, this invention provides a semiconductordevice that comprises a semiconductor substrate having a first majorsurface; an insulating layer optionally present on the first majorsurface of the substrate; and, disposed on the first major surface, aconductive electrode pattern having a preselected configuration andformed by firing a paste composition as described above.

A semiconductor device fabricated as described above may be incorporatedinto a photovoltaic cell. In another embodiment, this invention thusprovides a photovoltaic cell array that includes a plurality of thesemiconductor devices as described, and made as described, herein.

Lightly Doped Emitter (LDE) Wafers

The present paste composition is useful in constructing photovoltaiccells using conventional, highly doped emitter (HDE) wafers, as well asso-called “lightly doped emitter” (LDE) wafers.

Si solar cells are made by adding controlled impurities (called dopants)to purified Si. Different dopants impart positive (p-type) and negative(n-type) semiconducting properties to the Si. The boundary (junction)between the p-type and n-type Si has an associated (built-in) voltagethat provides power to electrical charge carriers in the solar cell.Dopant concentration must be controlled to achieve optimal cellperformance. A high dopant concentration in the emitter ordinarilyimparts low electrical emitter sheet resistivity and enables a lowresistivity metal contact to be made at the Si surface, therebydecreasing resistance losses. However, a high dopant concentration alsomay introduce crystalline defects or electrical perturbations in the Silattice that increase recombination losses.

A common Si solar cell design comprises a ˜200 micron thick p-type Siwafer coated with a 0.4 micron thick n-type Si layer. The p-type waferis the base. The n-type layer is the emitter. This configuration istypically made by either diffusion or ion implantation of phosphorus (P)dopant into the Si wafer.

Typical highly doped Si emitters (HDE) have total [P_(surface)] rangingfrom 9 to 15×10²⁰ atoms/cm³ and active [P_(surface)] ranging from 3 to4×10²⁰ atoms/cm³. Lightly doped emitters have total [P_(surface)]ranging from 0.9 to 2.9×10²⁰ atoms/cm³ and active [P_(surface)] rangingfrom 0.6 to 2.0×10²⁰ atoms/cm³. P dopant in excess of the activeconcentration (inactive P) leads to Shockley-Read-Hall (SRH)recombination energy loss. Active P dopant above 1×10²⁰ atoms/cm³ leadsto Auger recombination energy loss.

Total dopant concentration is typically measured using the SIMS(secondary ion mass spectrometry) depth profiling method [Diffusion inSilicon, S. W. Jones, IC Knowledge LLC 2008, pages 56-62; see page 61].Active dopant concentration is often measured using SRP (spreadingresistance probing) or ECV (electrochemical capacitance voltage)methods.

Solar cell embodiments employing lightly doped emitters in someinstances achieve improved solar cell performance by decreasing thelosses resulting from electron-hole recombination at the front surface.However, the inherent potential of LDE-based cells to provide improvedcell performance often is not fully realized in practice because of thegreater difficulty of forming the high-quality metal contacts needed toefficiently extract current from the operating cell.

As a result, wafers used for commercial solar cells heretofore havetypically employed high [P_(surface)] emitters, as discussed above,which degrade short wavelength response (short wavelengths having a veryhigh absorption coefficient in silicon and are absorbed very close tothe surface) and result in lower open-circuit voltage V_(oc) andshort-circuit current density J_(sc). The high [P_(surface)] emittersenable formation of low contact resistivity metallization contacts,without which contact is poor and cell performance is degraded.

Nevertheless, there remains an improvement in cell performancepotentially attainable with LDE-based cells. Such cells would require athick-film metallization paste that can reliably contact lightly doped,low [P_(surface)] emitters without damaging the emitter layer surface,while still providing low contact resistance. Ideally, such a pastewould enable screen-printed crystalline silicon solar cells to havereduced saturation current density at the front surface (J_(0e)) andaccompanying increased V_(oc) and J_(sc), and therefore improved solarcell performance. Other desirable characteristics of a paste wouldinclude high bulk conductivity, the ability to form narrow,high-aspect-ratio finger lines in a metallization pattern to furtherreduce series resistance and minimize shading of incident light by theelectrodes, and good adherence to the substrate.

EXAMPLES

The operation and effects of certain embodiments of the presentinvention may be more fully appreciated from Examples 1-11 describedbelow. The embodiments on which these examples are based arerepresentative only, and the selection of those embodiments toillustrate aspects of the invention does not indicate that materials,components, reactants, conditions, techniques, and/or configurations notdescribed in the examples are not suitable for use herein, or thatsubject matter not described in the examples is excluded from the scopeof the appended claims and equivalents thereof.

Examples 1a and 2a Paste Preparation

In accordance with the present disclosure, alkaline-earth-metal boronbismuth oxide materials as set forth in Table I were prepared. Thecompositions were formulated by combining requisite amounts of thecompounds BaCO₃, CaCO₃, Bi₂O₃, B₂O₃, and SiO₂. The amount of eachcompound was selected to provide in the combined alkaline-earth-metalboron bismuth oxide the cation percentages listed in Table I.

The various ingredients for each composition were intimately mixed bymelting them in a covered Pt crucible that was heated in air from roomtemperature to 1100° C. over a period of 1 hour, and held at thattemperature for 30 minutes. Each melt was separately poured onto theflat surface of a cylindrically-shaped stainless steel block (8 cm high,10 cm in diameter). The cooled buttons were pulverized to a −100 meshcoarse powder.

Then the coarse powder was ball milled in a polyethylene container withzirconia media and a suitable liquid, such as water, isopropyl alcohol,or water containing 0.5 wt. % TRITON™ X-100 octylphenol ethoxylatesurfactant (available from Dow Chemical Company, Midland, Mich.) untilthe d₅₀ was in the range of 0.5 to 2 μm.

TABLE I Alkaline-earth-metal Boron Bismuth Oxide Material Compositionscation % cation % cation % cation % cation % Example # B Bi Si Ba Ca 1a50.00 12.50 6.25 18.75 12.50 2a 53.33 26.67 6.67 6.67 6.67

In accordance with an aspect of the invention, the alkaline-earth-metalboron bismuth oxide compositions of Examples 1a and 2a were combinedwith silver powder and an organic vehicle to form paste compositionssuitable for screen printing.

The silver powder used was represented by the manufacturer as having apredominantly spherical shape. The powder was found to have a particlesize distribution with a d₅₀ of about 2.3 μm by measurement in anisopropyl alcohol dispersion using a Horiba LA-910 analyzer.

The organic vehicle was prepared as a master batch using a planetary,centrifugal Thinky mixer (available from Thinky USA, Inc., Laguna Hills,Calif.) to mix the ingredients listed in Table II below, withpercentages given by weight.

TABLE II Organic Vehicle Composition Ingredient wt. % 11% ethylcellulose (50-52% ethoxyl) 13.98 dissolved in TEXANOL ™ solvent 8% ethylcellulose (48-50% ethoxyl) 5.38 dissolved in TEXANOL ™ solventtallowpropylenediamine dioleate 10.75 pentaerythritol ester ofhydrogenated 26.88 rosin hydrogenated castor oil derivative 5.38 dibasicester 37.63

The paste compositions were formulated by combining approximately 9.7wt. % vehicle and 2, 3, or 4 wt. % of the alkaline-earth-metal boronbismuth oxide materials of either Example 1a or Example 2a, with theremainder being silver powder. First, the milled oxide material andsilver powder was combined in a glass jar and tumble mixed for 15minutes. This inorganic mixture was then added by thirds to a Thinky jarcontaining the organic ingredients and Thinky-mixed for 1 minute at 2000RPM after each addition. After the final addition, the paste was cooledand the viscosity was adjusted to between about 300 and 400 Pa-s byadding a suitable small portion of TEXANOL™ solvent and Thinky mixingfor 1 minute at 2000 RPM. Viscosities herein were measured with aBrookfield viscometer (Brookfield Inc., Middleboro, Mass.) with a #14spindle and a #6 cup. Viscosity values were taken after 3 minutes at 10RPM.

The paste was then milled on a three-roll mill (Charles Ross and Son,Hauppauge, N.Y.) with a 25 μm gap for 3 passes at zero pressure and 3passes at 100 psi (689 kPa). The paste was allowed to sit overnight, andthen its viscosity was adjusted to approximately 300 Pa-s with smalladditions of solvent, if necessary.

Examples 1b and 2b Fabrication and Testing of Photovoltaic Cells CellFabrication

Photovoltaic cells were fabricated in accordance with an aspect of theinvention using the paste compositions made with the oxides of Examples1a and 2a at different loadings to form the front-side electrodes forthe cells of Examples 1b and 2b. Examples 1b(1)-1b(3) and 2b(1)-2b(3)were made with different loadings of the frits of Examples 1a and 2a,respectively. The amount of frit in each composition is listed in TableIII as a weight percentage based on the total paste composition.

Conventional HDE Deutsche Cell multi-crystalline wafers (˜200 μm thick,˜65 ohms per square resistivity) were used for fabrication andelectrical testing. For convenience, the experiments were carried outusing 28 mm×28 mm “cut down” wafers prepared by dicing 156 mm×156 mmstarting wafers using a diamond wafering saw. The test wafers werescreen printed using an AMI-Presco (AMI, North Branch, N.J.) MSP-485screen printer, first to form a full ground plane back-side conductorusing a conventional Al-containing paste, SOLAMET® PV381 (available fromDuPont, Wilmington, Del.), and thereafter to form a bus bar and elevenconductor lines at a 0.254 cm pitch on the front surface using thevarious exemplary paste compositions herein. After printing and drying,cells were fired in a BTU rapid thermal processing, multi-zone beltfurnace (BTU International, North Billerica, Mass.). Twenty five cellswere printed using each paste; 5 cells were fired at each set pointtemperature in a 5-temperature ladder ranging from set points 880 to940° C. After firing, the median conductor line width was about 110 μmand the mean line height was about 15 μm. The bus bar was 1.25 mm wide.Performance of “cut-down” 28 mm×28 mm cells is known to be impacted byedge effects which reduce the overall photovoltaic cell efficiency by˜5% from what would be obtained with full-size wafers.

Electrical Testing

Electrical properties of photovoltaic cells as thus fabricated weremeasured at 25±1.0° C. using an ST-1000 IV tester (Telecom STV Co.,Moscow, Russia). The Xe arc lamp in the IV tester simulated sunlightwith a known intensity and irradiated the front surface of the cell. Thetester used a four contact method to measure current (I) and voltage (V)at approximately 400 load resistance settings to determine the cell'sI-V curve. Efficiency, fill factor (FF), and series resistance (R_(a))were obtained from the I-V curve for each cell. R_(a) is defined in aconventional manner as the negative of the reciprocal of the local slopeof the IV curve near the open circuit voltage. As recognized by a personof ordinary skill, R_(a) is conveniently determined and a closeapproximation for R_(s), the true series resistance of the cell. Foreach composition, an optimum firing temperature was identified as thetemperature that resulted in the highest median efficiency, based on the5-cell test group for each composition and temperature. Electricalresults for the cell groups fired at the respective optimal firingtemperature are depicted in Table III below. Of course, this testingprotocol is exemplary, and other equipment and procedures for testingefficiencies will be recognized by one of ordinary skill in the art.

TABLE III Electrical Properties of Multi-crystalline Photovoltaic Cellswt. % frit Eff. FF Ra Example # in paste (%) (%) (ohms) 1b(1) 2 13.9666.3 0.3614 1b(2) 3 13.84 71.9 0.3104 1b(3) 4 13.77 67.9 0.3982 2b(1) 29.46 47.9 1.0443 2b(2) 3 12.80 62.6 0.4936 2b(3) 4 14.44 72.0 0.2992

Examples 3a to 11a Paste Preparation

Using the same melting, quenching, and milling procedures employed forExamples 1a and 2a, further alkaline-earth-metal boron bismuth oxidematerials in accordance with the present disclosure were prepared, asset forth in Table IV.

TABLE IV Alkaline-earth-metal Boron Bismuth Oxide Material CompositionsExample cation % cation % cation % cation % cation % cation % cation %cation % cation % # B Bi Ba Ca Li P Ti Zn Si  3a 45.01 12.50 23.99 6.014.99 7.50 0.00 0.00 0.00  4a 44.98 12.50 20.00 5.01 5.01 7.50 5.01 0.000.00  5a 44.99 12.50 20.00 4.99 5.04 7.49 0.00 4.99 0.00  6a 50.00 12.5018.75 12.50 0.00 0.00 0.00 0.00 6.25  7a 53.33 26.67 6.67 6.67 0.00 0.000.00 0.00 6.67  8a 32.00 40.00 18.00 5.00 5.00 0.00 0.00 0.00 0.00  9a32.00 40.00 9.00 5.00 5.00 0.00 0.00 9.00 0.00 10a 14.40 57.60 18.005.00 5.00 0.00 0.00 0.00 0.00 11a 14.40 57.60 9.00 5.00 5.00 0.00 0.009.00 0.00

The alkaline-earth-metal boron bismuth oxide compositions of Examples 3ato 11a were combined with the same silver powder and organic vehicle andprocessed as described for Examples 1a and 2a to form paste compositionssuitable for screen printing.

Examples 3b to 11b Fabrication and Testing of Photovoltaic Cells

Photovoltaic cells were fabricated and tested using the techniquesgenerally described above for Examples 1b and 2b. For Examples 3b to 5b,paste compositions were prepared with 3.5 wt. % loading of the frits ofExamples 3a to 5a, respectively. For Examples 6b to 11b, pastecompositions were prepared at multiple loadings of the frits of Examples6a to 11a, respectively. Each paste composition was then used to printfront-side electrodes on ˜200 μm thick, ˜65 ohms per square resistivity,HDE mono-crystalline wafer substrates (available from Gintech EnergyCorporation, Jhunan Township, Taiwan). Cells were again prepared andtested on 28 mm×28 mm “cut down” wafers, using the same firing andtesting protocols as before.

Electrical properties obtained at the optimal firing temperature foreach composition are set forth in Table V. The data demonstrate thatoperable photovoltaic cells can be fabricated with the pastes ofExamples 3a to 11a.

TABLE V Electrical Properties of Mono-crystalline Photovoltaic Cells wt.% frit Eff. FF Ra Example # in paste (%) (%) (ohms) 3b(1) 3.5 10.28 48.10.7289 4b(1) 3.5 13.38 60.8 0.5025 5b(1) 3.5 13.99 64.7 0.4193 6b(1) 213.96 66.3 0.3614 6b(2) 3 13.84 71.9 0.3104 6b(3) 4 13.77 67.9 0.39827b(1) 2 9.46 47.9 1.0443 7b(2) 3 12.80 62.6 0.4936 7b(3) 4 14.44 72.00.2992 8b(1) 2 14.49 64.3 0.3761 8b(2) 3 15.20 68.0 0.3272 9b(1) 2 15.1966.8 0.3202 9b(2) 3 14.86 66.0 0.3919 10b(1)  2 15.28 67.6 0.340310b(2)  3 13.88 62.8 0.4320 11b(1)  2 15.29 69.4 0.3151 11b(2)  3 15.4269.2 0.3255

Having thus described the invention in rather full detail, it will beunderstood that this detail need not be strictly adhered to but thatfurther changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the invention as defined bythe subjoined claims.

Where a range of numerical values is recited or established herein, therange includes the endpoints thereof and all the individual integers andfractions within the range, and also includes each of the narrowerranges therein formed by all the various possible combinations of thoseendpoints and internal integers and fractions to form subgroups of thelarger group of values within the stated range to the same extent as ifeach of those narrower ranges was explicitly recited. Where a range ofnumerical values is stated herein as being greater than a stated value,the range is nevertheless finite and is bounded on its upper end by avalue that is operable within the context of the invention as describedherein. Where a range of numerical values is stated herein as being lessthan a stated value, the range is nevertheless bounded on its lower endby a non-zero value.

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage, where an embodiment of thesubject matter hereof is stated or described as comprising, including,containing, having, being composed of, or being constituted by or ofcertain features or elements, one or more features or elements inaddition to those explicitly stated or described may be present in theembodiment. An alternative embodiment of the subject matter hereof,however, may be stated or described as consisting essentially of certainfeatures or elements, in which embodiment features or elements thatwould materially alter the principle of operation or the distinguishingcharacteristics of the embodiment are not present therein. A furtheralternative embodiment of the subject matter hereof may be stated ordescribed as consisting of certain features or elements, in whichembodiment, or in insubstantial variations thereof, only the features orelements specifically stated or described are present. Additionally, theterm “comprising” is intended to include examples encompassed by theterms “consisting essentially of” and “consisting of.” Similarly, theterm “consisting essentially of” is intended to include examplesencompassed by the term “consisting of.”

When an amount, concentration, or other value or parameter is given aseither a range, preferred range, or a list of upper preferable valuesand lower preferable values, this is to be understood as specificallydisclosing all ranges formed from any pair of any upper range limit orpreferred value and any lower range limit or preferred value, regardlessof whether ranges are separately disclosed. Where a range of numericalvalues is recited herein, unless otherwise stated, the range is intendedto include the endpoints thereof, and all integers and fractions withinthe range. It is not intended that the scope of the invention be limitedto the specific values recited when defining a range.

In this specification, unless explicitly stated otherwise or indicatedto the contrary by the context of usage,

(a) amounts, sizes, ranges, formulations, parameters, and otherquantities and characteristics recited herein, particularly whenmodified by the term “about”, may but need not be exact, and may also beapproximate and/or larger or smaller (as desired) than stated,reflecting tolerances, conversion factors, rounding off, measurementerror, and the like, as well as the inclusion within a stated value ofthose values outside it that have, within the context of this invention,functional and/or operable equivalence to the stated value; and

(b) all numerical quantities of parts, percentage, or ratio are given asparts, percentage, or ratio by weight; the stated parts, percentage, orratio by weight may or may not add up to 100.

What is claimed is:
 1. A paste composition comprising: (a) a source of electrically conductive metal; (b) an alkaline-earth-metal boron bismuth oxide; and (c) an organic vehicle, in which the source of electrically conductive metal and the oxide are dispersed.
 2. The paste composition of claim 1, comprising 0.5 to 10 weight % of the alkaline-earth-metal boron bismuth oxide.
 3. The paste composition of claim 1, wherein alkaline-earth metal, boron, and bismuth cations comprise 75 to 95 cation % of the alkaline-earth-metal boron bismuth oxide.
 4. The paste composition of claim 1, wherein the alkaline-earth-metal boron bismuth oxide comprises: 10 to 40 cation % of an alkaline earth metal selected from the group of Mg, Ca, Ba, Sr, and mixtures thereof; 14 to 65 cation % of B; and 10 to 60 cation % of Bi, plus incidental impurities.
 5. The paste composition of claim 4, wherein the alkaline-earth-metal boron bismuth oxide further comprises at least one oxide selected from the group consisting of oxides of Al, Li, Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Si, Mo, W, Hf, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, P, Y, La and the other lanthanide elements, and mixtures thereof.
 6. The paste composition of claim 5, wherein the alkaline-earth-metal boron bismuth oxide further comprises at least one oxide selected from the group consisting of oxides of Li, Na, Si, P, Zn, and Ti.
 7. The paste composition of claim 6, wherein the alkaline-earth-metal boron bismuth oxide further comprises: 0 to 15 cation % of Li; 0 to 15 cation % of Na; 0 to 15 cation % of Si; 0 to 15 cation % of P; 0 to 20 cation % of Zn; and 0 to 20 cation % of Ti, plus incidental impurities.
 8. The paste composition of claim 1, wherein up to 10 anion percent of the oxygen anions of the alkaline-earth-metal boron bismuth oxide are replaced by halogen anions.
 9. The paste composition of claim 1, wherein the source of the electrically conductive metal is an electrically conductive metal powder.
 10. The paste composition of claim 1, wherein the electrically conductive metal comprises Ag.
 11. The paste composition of claim 10, wherein the Ag comprises 85 to 99.5 wt. % of the solids in the composition.
 12. The paste composition of claim 1, wherein the paste composition is lead-free.
 13. The paste composition of claim 1, further comprising an oxide additive that is an oxide of Al, Li, Na, K, Rb, Cs, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Si, Mo, W, Hf, Ag, Ga, Ge, In, Sn, Sb, Se, Ru, Bi, Ba, Ca, Sr, Mg, B, P, Y, La or the other lanthanide elements, or mixtures thereof, or a compound of one or more of the above elements which form an oxide upon firing.
 14. A process for forming an electrically conductive structure on a substrate, the process comprising: (a) providing a substrate having a first major surface; (b) applying a paste composition onto a preselected portion of the first major surface, wherein the paste composition comprises in admixture: i) a source of electrically conductive metal, ii) an alkaline-earth-metal boron bismuth oxide, and iii) an organic vehicle, in which the source of electrically conductive metal and the oxide are dispersed; and (c) firing the substrate and paste composition thereon, whereby the electrically conductive structure is formed on the substrate.
 15. The process of claim 14, wherein the source of electrically conductive metal is silver powder.
 16. The process of claim 14, wherein the substrate comprises an insulating layer present on at least the first major surface and comprising at least one layer comprised of aluminum oxide, titanium oxide, silicon nitride, SiN_(x):H, silicon oxide, or silicon oxide/titanium oxide, the paste composition is applied onto the insulating layer of the first major surface, and the insulating layer is penetrated and the electrically conductive metal is sintered during the firing, whereby an electrical contact is formed between the electrically conductive metal and the substrate.
 17. An article comprising a substrate and an electrically conductive structure thereon, the article having been formed by the process of claim
 14. 18. The article of claim 17, wherein the substrate is a silicon wafer.
 19. The article of claim 17, wherein the article comprises a semiconductor device.
 20. The article of claim 19, wherein the article comprises a photovoltaic cell. 